The surface of a cylinder bore may be end-processed by means of, for example, a honing method. Description of the resultant surfaces may include roughness values from one or more roughness standards. For example roughness values Rpk, Rk and Rvk, are disclosed in DIN 4776. Other standards may be used.
A low roughness value may shorten the run-in time of the internal combustion engine, and may reduce the oil consumption and may reduce the emissions as a result of burnt oil. The friction between the piston or the piston rings thereof and the bore surface, which may be referred to as the slide way, may also be reduced, which can have a positive effect on the fuel consumption, and consequently on the CO2 emissions.
The roughness values Rpk and Rk may be as low as possible, wherein the roughness value Rvk is selected so as to be dependent on the resistance stability and consequently the robustness and reliability of the internal combustion engine. This means that the roughness value Rvk may be mainly responsible for the quantity of the oil retained in the indentations in order to lubricate this tribological system adequately. The magnitude of the Rvk value should consider and ensure the behavior of the contact surfaces over the entire service-life of the internal combustion engine so that a precise function is provided over the entire service-life.
Therefore, current internal combustion engines may have at the bore surface thereof by way of example the following roughness values: Rpk=0.0-0.35 μm/Rk=0.2-0.9 μm/Rvk=0.8-2.3 μm. These values are typically intended to be identical over the entire bore surface.
However, it has been found that during a piston stoppage, that is to say, in the top and bottom dead center, the hydrodynamic behavior is cancelled and a mixed friction occurs. Therefore, there are known attempts to vary the roughness value over the bore surface. A higher roughness value Rvk may be provided at the top and bottom dead center portion than in an intermediate portion arranged therebetween. This may cause a higher oil storage in the rougher surface so that the mixed friction and therefore the wear may be reduced. In the intermediate portion in which the piston has the greatest relative speed with respect to the bore surface, however, the roughness value Rvk may be reduced as a result of the hydrodynamic behavior.
For example, DE 695 05 467 T2 discloses a cylinder having different surface states, wherein the cylinder bore surface has been honed. The cylinder bore surface of DE 695 05 467 T2 has, when viewed in a vertical direction, three different portions, of which only one is finely honed. An upper portion, that is to say, a top dead center portion, and a lower portion, that is to say, a bottom dead center portion of the cylinder bore surface, is coarsely honed, respectively. A center portion which is arranged therebetween is finely honed. The deep indentations of the coarsely honed surfaces obtained allow lubricant to be retained therein, whereby the wear is reduced. On the smoothly honed center portion at which the piston speed is relatively high relative to the cylinder bore surface, a hydrodynamic lubrication is provided.
The inventors herein have recognized a number of shortcomings with this approach. For example, the level of roughness on counter-pressure side at the upper and lower ends of the bore may be greater than necessary. These areas may tend to adversely affect the overall smooth movement of the piston. The present disclosure may provide a cylinder bore configured to receive a piston for reciprocating movement therein. The cylinder bore may have a pressure side and a counter-pressure side, and may include a bore surface having a first roughness value in a first longitudinal region on the pressure side, and a second roughness value different from the first roughness value in the first longitudinal region on the counter-pressure side. In this way, the cylinder bore may have improved tribological properties.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.